One of the attributes of a key telephone system is that the line circuit is arranged to provide lamp signals indicative of the status of the associated stations. These lamp signals are ac signals having various distinctive duty cycles. The lamp wink signal used for the hold state and the lamp flash signal used for the ringing state are examples of the different signals provided. At present an interrupter circuit common to all of the line circuits provides the proper voltage signals, and relay contacts in each of the line circuits extend the commonly generated voltage signals to the associated lamp.
In the absence of relay contacts in the line circuit, as for example, when the line circuit is designed using solid state devices, provision must be made to extend the ac lamp signals to the associated stations. One possible solution to the problem would be to internally generate the lamp signals using dc signals and to apply the generated signals to the lamp. This approach, while solving the problem of switching ac signals, has the major disadvantage that the signals generated at each line circuit would be out of sync with each other and thus when a station is served from more than one line circuit the lamps associated with each line circuit could pulsate out of phase with each other thereby creating a highly undesirable situation.
Another replacement for the relay contact for switching ac power is the ac switch or triac. Typically, such a triac would be connected into the circuit in much the same manner as would the relay contact and the triac would then be operated by a control signal. Undersuch a condition there would be required a single triac for each of the various duty cycles.
Thus, it is desired to arrange a lamp control circuit capable of selectably providing a number of different signal rates to one or more lamps, all under control of a digital signal corresponding to the desired rate.